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infant (Photo credit: soupboy)

Autism risk might be cut nearly in half by making sure that mothers are taking supplements containing folic acid, a B vitamin, at the time of conception and in the early months of pregnancy.

“This is important because this is something women can do to reduce the risk of autism,” says Alycia Halladay, Senior Director of Clinical and Environmental Sciences at Autism Speaks, an advocacy group.

The study, published in the new issue of the Journal of the American Medical Association, followed 85,176 children born in Norway. Only 114, or 0.13%, of those children had autistic disorder. But 0.2% of the children whose mothers did not take folic acid had autism, compared to 0.1% of the children of mothers who took the vitamin in pill form. All told, women who took folic acid supplements before and early in pregnancy were 39% less likely to have autistic children. Stated differently, of every 10 children who would have become autistic without the folic acid supplements, 6 would be autistic with them.

There was, however, no relationship between folic acid and Asperger's syndrome, a related condition, or pervasive developmental disorder, perhaps because those conditions were rare and there were not enough cases to show a difference.

“It doesn't prevent autism,” says Halladay. “Clearly some women who take folic acid will go on to have an autistic child. But it is something that women can do and can feel empowered to do.”

There are also factors that increase the risk of autism, Halliday says, including infections early in pregnancy, premature birth, and pesticide and chemical exposure.

Importantly, the effect was only apparent if mothers took the folic acid supplements during the six weeks before and the six weeks after conception. Folic acid supplementation is already recommended to prevent neural tube defects, which are often severe malformations of the brain.

A single study of this type is not definitive, no matter how large it was. But similar results had already been seen in a California study. It recruited patients who had already been diagnosed with autism, instead of looking at a vast number of births, but arrived at nearly the same 40% decrease in risk. “This adds to the already strong reasons for pre-pregnant women to be taking folic acid,” says Irva Hertz-Picciotto, chief of the division of environmental and occupational health at UC-Davis, one of the California effort's authors.

Pal Suren, the lead author of the Norwegian study, says that no adjustments for parental age or other factors seemed to make a big difference in the size of the effect. Moreover, other vitamin supplements, like fish oil pills, were not related to autism rates. That's important because if the effect were caused by the mother's being wealthier or more educated and getting better medical care, the fish oil pills would probably have looked beneficial, too.

Other studies, Suren says, back up the idea that nutritional deficiencies early in development can cause brain disorders. Another analysis from the same Norwegian effort showed that children who did not get folic acid were more likely to have trouble learning to speak. And an analysis of children who had faced extreme hunger after World War II were more likely to develop schizophrenia.

In addition to a way of reducing risk, this result could be a clue for further research. Ricardo Dolmetsch, a researcher at Stanford University and the Allen Institute for Brain Sciences, is using stem cell technology to study how the neurons of people with autism differ from those of other people. Already, he says, there seems to be a biological linkage between autism and what seem to be very different diseases, like schizophrenia.

“What is most interesting in my opinion is that folic acid supplementation from four weeks before to eight weeks after pregnancy does not have a larger effect,” he writes. “ In other words it halves your chance of developing autism but the vast majority of mothers who did not get supplements nevertheless had normal babies.”

He wonders whether genetic differences in how people's bodies use folic acid might be one of the reasons autism occurs in the first place. If some mothers don't process folic acid well, they might need more of the supplement. At least it's a lead.

Women in Norway are prescribed 400 micrograms of folic acid daily; in the U.S., they are told to take 800 mcg. It's recommended that women who could get pregnant take the supplement every day, even if they are not planning to conceive.